An endless metal belt is wound around a pair of pulleys in CVT used as a transmission in automobiles and the like. This endless metal belt is formed such that plural endless belt-like metal rings, of which circumference lengths are slightly different from each other, are stacked in a thickness direction, a pair of stacked metal rings is engaged with each other from a width direction, and many plate-like elements are annularly disposed toward each other in a thickness direction.
The above metal ring is produced as follows. First, opposite ends of a thin plate made of a steel material of maraging steel, stainless steel, or the like are welded, so that the thin plate is formed into a cylindrical shape. In this case, since a portion of steel material becomes hardened by welding heat, this cylindrical steel material is subjected to solution treatment, thereby being homogenized. Next, the cylindrical steel material subjected to the solution treatment is cut by a predetermined width, endless belt-like rings are thereby formed, and each ring is subjected to rolling so as to have a predetermined thickness. Since metal structures of the rings are deformed by the rolling, the rings subjected to the rolling are subjected to solution treatment again, so that the metal structures are recrystallized. After that, each ring subjected to the solution treatment is adjusted so as to have a predetermined circumference length, and metal rings, of which circumference lengths are slightly different from each other, are produced. Each metal ring is subjected to aging treatment and nitriding treatment, so that hardness and wear resistance thereof are improved.
A gas nitriding method, a gas soft-nitriding method, a salt bath nitriding method, and or the like, in which metal rings are held in a predetermined atmosphere for a time period, have been known as the above nitriding treatment. In the gas nitriding method and the gas soft-nitriding method, the whole metal ring is desirably exposed to a predetermined atmosphere, so that the metal ring is uniformly nitrided.
In this nitriding treatment for metal rings, a jig is used for holding many metal rings and nitriding the metal rings simultaneously. Specifically, a nitriding treatment jig for metal rings has been proposed as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-161314. In this nitriding treatment jig, the metal rings are suspended from an inner periphery side to a vertical direction by a pair of suspending devices provided at an upper side and a lower side. However, since the metal rings are suspended in this nitriding treatment jig, the metal rings are unstable in a heating furnace. A filling rate of the metal rings in the heating furnace is low, so that the weight of the jig per the number of metal rings is large, and heating efficiency in heating is deteriorated. The load on the metal ring abutting on the upper side suspending device is concentrated, and deformation or the like of the metal ring occurs in the nitriding treatment.
A nitriding treatment jig for metal rings has been proposed as disclosed in, for example, Japanese Patent No. 4219186. In this nitriding treatment jig, a frame body, in which metal rings are insertably provided in a radial direction, is provided, the metal rings are pressed from a peripheral side by plural members of the frame body so as to be bent in an ellipsoidal shape, and it is held in this condition. However, since the metal rings are also held so as to have an ellipsoidal shape in a vertical direction in this nitriding treatment jig, deformation or the like of the metal ring occur in the nitriding treatment. The above deformation or the like may be corrected by correction of circumference length in a circumference length correction process. However, when the deformation or the like of the metal ring is large, the time period for performing correction of deformation or the like together with the correction of circumference length may be very long.
In order to solve the above problem, a transfer rack for metal rings has been proposed as disclosed in, for example, Japanese Patent No. 2008-240085. In this transfer rack for metal rings, three or more rods parallel to each other stand in a vertical direction, many ring seats, which have a double conical shape projecting in a horizontal direction, are provided to the rods, and the metal rings are inserted between the rods, so that each metal ring is held from a peripheral surface side thereof by a V-shaped recess formed by inclined surfaces of the ring seats next to each other.
However, since mounting of the metal rings to this transfer rack and removing of the metal rings therefrom are manually performed, it is time consuming to mount and remove of the metal rings in large numbers, and the number that can be produced is limited. Even if mounting and removing of the metal rings are automatically performed in the above conventional technique, in a case in which the metal rings are processed one by one, substantial time is consumed for completing the processing of all the metal rings. Due to this, the number produced will be limited or more equipment must be provided.